1. Field of the Invention
The present invention relates to an electrophotographic copier, laser printer, facsimile apparatus or similar image forming apparatus and more particularly to an image forming apparatus capable of detecting a jam that is ascribable to a recording medium wrapped around a heat roller
2. Description of the Background Art
An image forming apparatus of the kind described includes a fixing device for fixing a toner image formed on a paper sheet or similar recording medium. The fixing device usually includes a heat roller and a press roller pressed against each other and rotatable to fix the toner image on the paper sheet. The fixing device additionally includes a plurality of peelers for peeling off the paper sheet from the heat roller. The surface of the heat roller is fluoridized for promoting the separation of the tone, which is melted by the heat roller and therefore viscous.
Usually, when a paper sheet jams a path inside the fixing device, the jam is detected on the basis of the output of a sheet sensor responsive to the remaining part of the paper sheet. However, a jam ascribable to a paper sheet wrapped around the heat roller due to defective peeling is not detected. Let this kind of jam be referred to as a wrap jam hereinafter. Such a paper sheet is therefore not removed from the heat roller and causes another wrap jam to occur. More specifically, the operator of the apparatus cannot easily see the paper sheet wrapped around the heat roller and therefore often leaves the former on the latter. Further, the display of a wrap jam can be reset or canceled and therefore causes the wrap jam to repeatedly occur. When the apparatus again starts operating after a stop, the heat roller heats the paper sheet wrapped therearound, resulting in an offensive smell, fume and other troubles.
Japanese Patent Laid-Open Publication No. 7-31933, for example, proposes to cope with a wrap jam by using a reflection type sensor responsive to the quantity of reflection from the surface of the heat roller. An arrangement for sensing the quantity of reflection or ultraviolet ways has also been proposed in the past. The problem with these prior art schemes is that reflectance depends on the color of paper sheets used and is apt to bring about detection errors. Another problem is that the use of a reflection type sensor or ultraviolet rays increases the cost of the apparatus.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 7-287473.
It is an object of the present invention to provide an image forming apparatus capable of easily, surely detecting a wrap jam and a jam occurred in a fixing device with a low-cost configuration, and preventing such jams from repeatedly occurring.
In accordance with the present invention, in an image forming apparatus including a fixing device for fixing a toner image formed on a recording medium, the fixing device includes a rotatable fixing member configured to contact the toner image formed on the recording medium, and a pressing member pressed against the fixing member for conveying the recording medium, cooperation with the fixing member. A heat source heats the fixing member. A first temperature sensor senses the temperature of the surface of the fixing member in a contact range in which the fixing member is expected to contact the recording medium. A second temperature sensor senses the temperature of the surface of the fixing member in a non-contact range outside of the contact range. A controller controls current supply to the heat source in accordance with the temperature sensed by at least the first temperature sensor. A reference temperature setting section sets a reference temperature value on the basis of the temperatures that are sensed by the first and second temperature sensors when the surface of the fixing member is heated to a preselected temperature by the heat source. The controller calculates an actual temperature value on the basis of the temperatures that are sensed by said temperature sensors when the surface of the fixing member is heated to the preselected temperature. The controller then compares the actual temperature value and reference temperature value to thereby determine whether or not a jam ascribable to the recording medium wrapped around the fixing member has occurred. Subsequently, the controller interrupts the current supply to the heat source if the jam has occurred.